This invention relates generally to data processing systems and particularly to controllers which provide means for interconnecting peripheral units, also commonly referred to as input/output devices, and a general purpose digital computer. More specifically, this invention relates to a system of distributed peripheral-unit controllers, each of which is microprogrammed to control the transfer of data signals and the operations of a specific peripheral unit. Each controller is electrically interposed between that specific peripheral unit and a common multiplexer and is located near or at the corresponding specific peripheral unit to which it is connected.